1. Field of the Invention
The present invention relates to a method and apparatus for assessing the integrity of sealed packages and, more particularly, to detection of volatile substances which may leak from sealed packages.
2. Description of Related Art
A variety of materials or products are packaged to protect the materials or products from contact with an uncontrolled environment. Such packages are designed to be leak-free because contact of the contents of the package with the outside environment would result in degradation and/or alteration of the contents, e.g., by evaporation or chemical reaction. Production of leak-free packages is important to the pharmaceutical and sterile medical products industries as well as to the food and chemical industries.
Numerous techniques have been developed to seal packages and form air-tight or hermetic seals. For example, a plastic laminate may be heat sealed together along the edges of the package or an adhesive may be applied to the interior edges of a pouch and the two halves are pressed against each other. Many methods for sealing packages involving various materials are known to those with skill in the art.
Unfortunately, mass production of air-tight packages usually results in a certain percentage of leaking packages. In order to prevent distribution of such defective packages, it is necessary to have techniques for detecting defective leaking packages. One method for detecting leaks involves filling a vessel with water, sealing the vessel, creating a partial vacuum within the vessel and placing a package under the water. If the package is defective, gas contained in the package will leak out and bubbles of escaping gas will appear. While the so-called water bath method is sensitive, it is destructive to the package, messy, and capillary action can give false results.
Another method for detecting leaks involves testing for flow of a trace gas leaking from a package as is described in an article by Demorest, R. entitled Non-Destructive Leak Detection of Blister Packs and Other Sterile Medical Packages, Journal of Packaging Technology, Vol. 2, No. 5 (1988). As a trace gas, either carbon dioxide or helium may be introduced into the package headspace in one of three ways: (1) flushing the package and its contents with the gas prior to sealing; (2) forcing the gas into the package after sealing with a pressurized bomb; and (3) providing the gas as a normal component of the product or package.
After the gas has been introduced into the package a pressure differential is induced across the package wall thus causing the gas to leak from the package. After the gas has been allowed to leak from the package, room air is allowed to sweep the gas to a detection sensor, such as an infrared detector in the case of carbon dioxide or a mass spectrometer in the case of helium. Detection of carbon dioxide by the infrared detector requires a CO.sub.2 concentration of 100 parts per million (ppm) or more at the detector and leaks providing less than the required concentration may go undetected. The mass spectrometer used to detect helium leaks has several shortcomings including susceptibility to overloading and the requirement of a hard vacuum, i.e., if there is any contamination, such as by water, a hard vacuum cannot be drawn until all of the water has evaporated. Furthermore, it may not be practical for certain production techniques to incorporate a step involving exposure to a gas.
While the above described techniques may be useful for certain applications, there is a need for leak detection techniques of improved sensitivity and which avoid the shortcomings of existing protocols. The present invention relates to such an improved method and apparatus.